The purposes of the research planned are: (a) to determine whether or not the rod receptor system of the human retina can affect the sensitibity of the cone receptor system; (b) to investigate further the evidence of adaptation of human photoreceptors; (c) to continue definition of a quantitative model of the various aspects of the directionally sensitive properties of the retina, referred to as the Stiles-Crawford effect; and (d) to investigate non-linearities of human rod receptors. All observations will be based on psychophysical methods.